Instancing + modifier speed
No, as far as I know there are no optimizations in place for constructive modifiers like mirror or array.
You do get some benefits in terms of workflow since you only need to do "Half the editing" and you may also get a benefit in file size, since only one half of the mesh is stored on disk.

Other than that I don't think there are performance benefits from those modifiers since depending on the settings the meshes they generate may not even be totally identical.